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Thunderstorm Cracking the AQ Code Air Quality Forecast Team October 2015 Volume 1, Issue 4 The Genesis of a Thunderstorm: An About “Cracking the AQ Code” Arizona Perspective By: Michael Graves, ADEQ Air Quality Meteorologist In an effort to further Cumulonimbus clouds exist as the pinnacle of all cloud formation. They can rise to the heights of 30,000-50,000 feet or higher on ADEQ’s mission of occasion. They are also capable of generating large amounts of protecting and enhancing energy, manifested by some of the most violent weather the public health and phenomena experienced on earth. Once a cumulonimbus cloud environment, the Forecast produces lightning, it is considered a thunderstorm. But how does Team has decided to a thunderstorm form in the first place? And how do thunderstorms produce periodic, in-depth affect air quality in Arizona? In this edition of Cracking the AQ articles about various topics Code, we explore the genesis of thunderstorms in central Arizona related to weather and air and their impacts on air quality. quality. Our hope is that these articles provide you with a better understanding of Arizona’s air quality and environment. Together we can strive for a healthier future. We hope you find them useful! Figure 1: Cumulonimbus clouds behind mountains in Tucson, Arizona. Notice how the clouds flatten and spread out at their top as they reach the upper limit of the troposphere. Upcoming Topics… Temps/Inversions/ Photo by: Cindy Devin No Burn Days ENSO (A.K.A El Niño) All About Fog Volume 1, Issue 4 1 Publication No. N-15-40 Thunderstorm Ingredients Have you ever heard someone say, “I think a storm is brewing!” upon observing a darkening sky in the middle of the afternoon? Meteorologically-speaking, this statement is not far from the truth. Similar to when you make your preferred pot of coffee or tea, a thunderstorm is simply the result of several ingredients coming together in the same place, at the same time, and given enough time to interact. There are three necessary ingredients for thunderstorm formation: instability, moisture, and a source of lift. Each is discussed in this section. Instability For a thunderstorm to come to life, air needs to be able to rise. An air parcel (an invisible volume of air) rises when it is warmer (less dense) than the air around it and sinks when it is colder (more dense). For an air parcel to rise upward through the atmosphere, it must remain warmer than its surrounding environment, much like a hot air balloon as it gains altitude. When an air parcel is able to continually rise, the atmosphere is considered “unstable.” In other words, instability is present. Instability increases in the atmosphere when the earth’s surface is heated by solar energy, when low-level moisture increases, and/or when the temperature in the upper-levels of the atmosphere decreases. During Arizona’s summer monsoon season, instability increases over the lower deserts as low-level moisture increases in the presence of intense solar heating. In the winter, instability is often provided by Pacific upper-level troughs that bring in cooler air aloft (see Figure 6). Overall, greater instability enables ordinary cumulus clouds to grow into their cumulonimbus form. Figure 2: It takes instability for cumulus clouds to achieve this kind of vertical development. This photo was taken from atop ADEQ’s parking garage, looking south toward South Mountain on July 6, 2015. Photo by: Michael Graves Volume 1, Issue 4 2 Publication No. N-15-40 Moisture This ingredient is probably the most intuitive, because you can’t have clouds without moisture. Suppose the atmosphere has enough moisture and instability for thunderstorms. An air cools as it begins to rise. Eventually it cools to its dewpoint, the temperature at which it can no longer hold any more water vapor. As a result, its water vapor condenses onto very small particles already present in the atmosphere, forming cloud droplets. Now imagine countless cloud droplets forming and then rising further into the atmosphere. Sooner or later these cloud droplets climb high enough to freeze and eventually become ice crystals. The primary sources of moisture for Arizona include the eastern Pacific Ocean and the Gulf of California. During the fall, winter, and early spring, low pressure systems draw moisture from these sources and make it available for precipitation and sometimes thunderstorms. In the summer months, the monsoon flow pattern draws moisture from these sources, but also from additional places, including upper-level moisture from the Gulf of Mexico, and low-level moisture from the vegetated Sierra Madre Occidental Mountains in western Mexico. Figure 3 shows a map of the various possible sources of moisture for the monsoon. Figure 3: Sources of low-level moisture for the North American Monsoon. Moisture from the Southern Plains is possible when crop fields have received enough rain Source: National Weather Service Volume 1, Issue 4 3 Publication No. N-15-40 Source of Lift The ingredient necessary for a thunderstorm is a source of lift. This refers to any physical or meteorological mechanism that can initiate the rising motion of air parcels. Sources of lift that lead to thunderstorm development in Arizona include: mountains/terrain, outflows from preexisting/dying thunderstorms, atmospheric waves, locally strong winds in the jet stream, cold fronts, unequal heating at the surface, and a dry line (i.e., a sharp boundary between moist and dry air masses). Figure 4 illustrates some of these sources of lift. A. Mountains/terrain (Chiricahua National Monument) B. Dry line (boundary of monsoon moisture) C. Cold front (associated with Pacific trough) D. Upper-level wave (see caption) Figure 4: Common sources of lift in Arizona that can act as focus points for thunderstorm development, provided sufficient moisture and instability. In panel d, a small trough is analyzed over northwestern Mexico at 5 AM, Saturday, July 18, 2015. This wave moved northward and provided lift for storms that caused flash flooding in Wickenburg, AZ later in the afternoon. Sources: panel A - photo by Ken Bosma; panel B - University of Arizona; panel C - NEXLAB College of DuPage; panel D - NOAA. Volume 1, Issue 4 4 Publication No. N-15-40 Thunderstorm Life Cycle If each of these three ingredients is forecast to be present in an area, then there is a potential for thunderstorm formation. As a thunderstorm forms, it undergoes three distinctive stages: 1) The towering cumulus stage 2) The mature stage 3) The dissipating stage The towering cumulus stage is characterized by a vertically growing cumulus cloud in an unstable environment. The lifeline of the cumulus tower is the “updraft” that forms within the cloud due to the rising motion of air parcels, which feeds warm, moist air into the cloud. As long as the updraft is pumping warm, moist air into the cloud, it will be sustained. Eventually, water droplets within the cloud become heavy enough to escape the hold of the updraft and begin to fall as precipitation. The downward momentum of the precipitation then results in the development of a “downdraft” which is a plume of downward-moving air. Once the downdraft leaves the cloud base, it is further strengthened by cooling below the cloud base as rain evaporates. The downdraft eventually reaches the ground. The mature stage is marked by the presence of both the updraft and downdraft within the cloud. In this stage, the cloud continues to grow, eventually reaching the tropopause (the top of the troposphere,) about 7.5 miles above the ground. Since air above the tropopause is very stable, it acts as a cap, preventing further vertical development. This causes the cloud to spread out horizontally, creating a distinctive anvil shape. It is also during this stage that lightning, hail, and strong winds at the surface can occur. As the cool downdraft hits the ground, it spreads out in all directions. Eventually the “pool” of cold air at the surface cuts off the storm’s life-giving updraft. This ultimately ushers in the thunderstorm’s dissipating stage in which it weakens and finally dies out, leaving a remnant anvil. en.wikipedia.com Figure 5: An illustration of the three stages of an ordinary thunderstorm’s life cycle. This is a very generalized depiction of a thunderstorm’s life cycle. In the Desert Southwest, rain from thunderstorms may not even reach the ground as it fully evaporates in the dry air below the cloud base. Source: National Weather Service Volume 1, Issue 4 5 Publication No. N-15-40 Thunderstorm Seasonality for Arizona Thunderstorms can form during any time of the year if all the necessary ingredients are in place. In Arizona, these ingredients tend to come together most often during the summer monsoon season (learn more about the North American Monsoon here.) Because of the combination of both strong solar heating and moisture during an active monsoon period, towering cumulus and cumulonimbus clouds can be seen developing over the higher terrain of north, east, and southeastern Arizona almost on a daily basis. The lower deserts of Phoenix and Tucson can see their own thunderstorms if enough moisture makes its way to these areas or if large scale winds blow thunderstorms from higher terrain to the valleys. Thunderstorms are typically less common during the colder months since the atmosphere lacks instability due to the colder air at the surface. The best chance for thunderstorms to develop in the colder months is when an upper-level wave or Pacific trough moves over the Southwest (Figure 6). Troughs increase instability, initiate a low-level flow that taps into the Gulf’s moisture, and are usually accompanied by a cold front at the surface.
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